It is established in the art to secure specially constructed telltale elements or tags to merchandise which is likely to be pilfered, and it is known to electronically monitor the exits of stores and warehouses, etc., and even of zones within them where such merchandise is dispensed to ascertain that the tags are deactivated or detached in the manner provided for authorized removal of the merchandise. In the past various methods and apparatus along these lines have been employed, as recited in U.S. Pat. Nos. 3,895,368, 3,711,848, and 3,707,711, but many of these known methods and apparatus have limitations on their reliability, tolerance and sensitivity. Some are susceptible to false triggering by metallic structures coincidentally manifesting similar properties to the special tags. In some, proximity of the human body to the apparatus tends to mask the effect of the equipment and to interfere with reliable operation.
The limitation on the method and apparatus disclosed in the above patents are such that their respective systems have proved incapable of discerning with a high level of reliability whether a tab has been moved into a zone being monitored, i.e., a control zone, or is merely in proximity to it. This causes too many false alarms when there is no telltale tage actually in the control zone.
There is the further limitation on the method and apparatus disclosed in U.S. Pat. No. 3,895,368 because in that patent the frequencies selected for use therein were limited by an attempt to avoid frequencies that would be very susceptible to false triggering.